You need to do RA with DHCP; RA is how clients get the default gateway

Thank you cholzhauer, does the RA can be separate with DHCP? If there is any way to build the RA in Windows Server?

I'm planning to upgrade my router software version but as it's in use I am afraid any problems in upgrade may disconnected whole network, if the RA can be build in Windows Server (AD) would be much better.

Yeah, there's a way you can use Windows to advertise for you. I've used it once just to try it, but I don't remember off the top of my head how I did it. It involves the netsh commands, but that's about all I can remember; you'll have to ask Google, I'm sure it won't be too hard to find.